ABSTRACT
The effect of annealing both in the oxygen atmosphere and at room temperatures on physical properties such as the pseudogap (Δ*(T)) and excess conductivity (σ'(T)) of untwined YBa2Cu3O7-δ (YBCO) single crystal with a small deviation from oxygen stoichiometry is studied. It was revealed that as the charge carrier density, nf, increases, Ð¢Ñ also slightly increases, whereas the temperature of the pseudogap opening, T*, decreases noticeably, which is consistent with the phase diagram (PD) of cuprates. The excess conductivity in the vicinity of Tc is represented by the Aslamazov-Larkin and Hikami-Larkin fluctuation theories, illustrating the three-dimensional to two-dimensional (i.e. 3D-2D) crossover with an increase in temperature. The crossover temperature T0 determines the coherence length along the c axis is ξc(0) = 0.86 Å, that is 2.6 times larger than for optimally doped YBCO single crystals with defects. Taking into account the short coherence length in high-temperature superconductors, in the model of free charge carriers the phase relaxation time of fluctuating Cooper pairs is determined, τφ (100 K) = (4.55 ± 0.4) · 10-13 s, which is slightly (1.2 times) larger than in well-structured YBCO films, and as in films, does not depend on nf. It is shown that Δ*(T) at different annealing stages practically does not change its shape. As in the well-structured YBCO films, Δ*(T) demonstrates maximum at Tpair~124 K which depends weakly on nf. However, the maximum value of Δ*(Tpair) increases with increasing nf, as it follows from the PD of cuprates. Comparing the experimental data with the Peters-Bauer theory we estimated the density of local pairs
ABSTRACT
The effect of hydrostatic pressure up to P = 1.7 GPa on the fluctuation conductivity σ'(T) and pseudogap ∆*(T) in Y0.95Pr0.05Ba2Cu3O7-δ single crystal with critical temperature Ð¢Ñ = 85.2 K (at P = 0) was investigated. The application of pressure leads to the increase in Tc with dTc/dP = +1.82 KâGPa-1 while the resistance decreases as dlnρ(100 K)/dP = -(10.5 ± 0.2) %âGPa-1. Regardless of the pressure, in the temperature interval from Tc to T0 (~88 K at P = 0) the behaviour of σ'(T) is well described by the Aslamazov - Larkin (AL - 3D) fluctuation theory, and above the T0 by the Lawrence - Doniach theory (LD). The Maki-Thompson (MT - 2D) fluctuation contribution is not observed. This indicates the presence of structural defects in the sample induced by Pr. Here it is determined for the first time that when the pressure is applied to the Y1-xPrxBa2Cu3O7-δ single crystal, the pseudogap increases as dlnΔ*/dP = 0.17 GPa-1.
ABSTRACT
Most of superconductors in a magnetic field are penetrated by a lattice of quantized flux vortices. In the presence of a transport current causing the vortices to cross sample edges, emission of electromagnetic waves is expected due to the continuity of tangential components of the fields at the surface. Yet, such a radiation has not been observed so far due to low radiated power levels and lacking coherence in the vortex motion. Here, we clearly evidence the emission of electromagnetic waves from vortices crossing the layers of a superconductor/insulator Mo/Si superlattice. The emission spectra consist of narrow harmonically related peaks which can be finely tuned in the GHz range by the dc bias current and, coarsely, by the in-plane magnetic field value. Our findings show that superconductor/insulator superlattices can act as dc-tunable microwave generators bridging the frequency gap between conventional radiofrequency oscillators and (sub-)terahertz generators relying upon the Josephson effect.
